A technique has been heretofore known which comprises incorporating a sensitizing dye into a silver halide emulsion during the preparation of a silver halide light-sensitive material so that the light-sensitive wavelength range of the silver halide emulsion is widened to effect optical sensitization.
As the spectral sensitizing dyes to be used for this purpose there have heretofore been known many compounds. Examples of such spectral sensitizing dyes include cyanine dyes, melocyanine dyes and xanthene dyes as described in T. H. James, The Theory of the Photographic Process, 3rd ed., 1966, Macmillan, N.Y., pp. 198-228.
When used in place of an ordinary silver halide emulsion, these silver halide emulsions must not only widen the light-sensitive range of the silver halide emulsion but also meet the following requirements:
(1) a proper spectral sensitizing range must be given; PA1 (2) a high sensitizing efficiency must be given to obtain a sufficiently high sensitivity; PA1 (3) no fogging occurs; PA1 (4) a small dispersion of sensitivity due to temperature fluctuation upon exposure; PA1 (5) no advese interactions with other kinds of additives, e.g., stabilizer, fog inhibitor, coating aid, color developer; PA1 (6) no sensitivity fluctuation upon storage of a silver halide emulsion containing a sensitizing dye, especially at an elevated temperature and a high humidity; and PA1 (7) no sensitizing dyes are diffused into the other light-sensitive layers to inhibit color stain after development.
These requirements are important particularly when a silver halide emulsion for silver halide color photographic material is prepared.
However, despite many attempts, it is not easy to inhibit the sensitivity drop after the storage of unprocessed specimens to a sufficient extent. In particular, when a polymethylene dye having an oxidation potential of 0.60 or lower (V vs SCE) is used as a sensitizing dye, the sensitivity drop after the storage of the unprocessed specimens becomes great, making it difficult to obtain sufficient properties.
A methine dye containing as a substituent a saturated or unsaturated 5- to 7-membered ring including at least one nitrogen atom is reported in European Patent Application Disclosure (EP) 0372573. However, this nitrogen-containing 5- to 7-membered ring serves as a fog inhibitor in a silver halide light-sensitive material, and there are disclosed as the nitrogen atoms contained in the 5- to 7-membered ring only nitrogen atoms at least one of which is in the form of ##STR4## by tautomerism.
The nitrogen atom contained in the nitrogen-containing aromatic polycyclic compound of the present invention represented by Ar is neither in the form of ##STR5## by tautomerism nor serves as a fog inhibitor.
It has also been known that when applied to ordinary silver halide emulsion, such a sensitizing dye not only widens the sensitive wavelength range of the emulsion but also changes the fluctuation in sensitivity of the emulsion during prolonged storage, particularly storage under elevated temperature and high humidity conditions.
However, despite many trials, the sensitivity drop.due to prolonged storage cannot be inhibited to a fully satisfactory extent.
On the other hand, products for use in a market which demands finishing a large number of prints at quick delivery date, e.g., light-sensitive materials for color photographic paper, are required to be rapidly processed. It has been well known that when the silver halide emulsion to be incorporated in these light-sensitive materials has a higher silver chloride content, the developing speed is drastically improved.
However, it has also been known that when an emulsion having a high silver chloride content is used, it causes much fog, making it difficult to obtain a high sensitivity.